What the body feels is every bit as significant as what the mind thinks, a neuroscientist argues. Turn to emotions to explain human consciousness and cultures

Nietzsche would have given four cheers for this intricately argued book, which is at once scientifically rigorous and humanely accommodating, and, so far as this reviewer can judge, revolutionary. Antonio Damasio, a professor of neuroscience, psychology and philosophy, sets out to investigate "why and how we emote, feel, use feelings to construct our selves ... and how brains interact with the body to support such functions". We are not floating seraphim, he reminds us, but bodies that think - and all the better for it.

From Plato onwards, western philosophy has favoured mind over "mere" body, so that by the time we get to Descartes, the human has become hardly more than a brain stuck atop a stick, like a child's hobbyhorse. This is the conception of humanness that Damasio wishes to dismantle. For him, as for Nietzsche, what the body feels is every bit as significant as what the mind thinks, and further, both functions are inextricably intertwined. Indeed, from the very start, among the earliest primitive life forms, affect - "the world of emotions and feelings" - was the force that drove unstoppably towards the flowering of human consciousness and the creation of cultures, Damasio insists.

The idea on which he bases his book is, he tells us, simple: "Feelings have not been given the credit they deserve as motives, monitors, and negotiators of human cultural endeavours." In claiming simplicity, it is possible the author is being a mite disingenuous. The tone in which he sets out his argument is so carefully judged, so stylistically calm and scientifically collected, that most readers will be lulled into nodding agreement. Yet a moment's thought will tell us that we conduct our lives largely in contradiction of his premise, and for the most part deal with each other, and even with ourselves, as if we were pure spirit accidentally and inconveniently shackled to half a hundredweight or so of forked flesh.

"Feelings, and more generally affect of any sort and strength," Damasio writes, "are the unrecognised presences at the cultural conference table." According to him, the conference began among the bacteria, which - who? - even in their "unminded existence ... assume what can only be called a sort of 'moral attitude'". In support of his claim, he adduces the various ways in which bacteria behave that bear a striking resemblance to human social organisation. The implication is, then, that "the human unconscious literally goes back to early life-forms, deeper and further than Freud or Jung ever dreamed of". Damasio's argument is that we are directly descended not only from the apes, but from the earliest wrigglers at the bottom of the primordial rock pool.

The keyword throughout the book is homeostasis, of which he offers a number of definitions, the clearest of which is the earliest, and which he favours enough to set it in italics: homeostasis is the force - the word seems justified - that ensures that "life is regulated within a range that is not just compatible with survival but also conducive to flourishing, to a projection of life into the future of an organism or a species".

Damasio, whose books include The Feeling of What Happens and Self Comes to Mind, is a scientist but also a convinced, one might say a crusading, humanist. He wants us to recognise the richness of life in all its aspects, good or bad; but he is no sentimentalist. The human condition is one of struggle and assertion and the will to prevail: "Life comes equipped with a precise mandate: resist and project life into the future, no matter what." Here again the shadow, or the radiance, of Nietzsche's thinking falls across the page.

Also called to the table is Spinoza - on whom Damasio has written at length - and his emphasis on conatus, the essential force by which all things strive to persevere, and which had for Spinoza the same significance that homeostasis has for Damasio.

There are echoes here too of William James, that most endearing of philosophers, as when Damasio pauses for a brief, Jamesian consideration of the anomalous fact that for all the hi-tech sophistication of modern life, we still cling to the primitive pleasure and reassurance of the domestic fireplace. And James would have been delighted by Damasio's "everydayness", his readiness to acknowledge the fundamental underpinnings of even our highest endeavours, for instance when he remarks in wonderment: "It is intriguing to think that the enteric nervous system" - that is, the gut - "might well have been the very first brain."

But Damasio, while ever ready to salute his predecessors and peers, is wholly his own man, and The Strange Order of Things is a fresh and daring effort to identify the true spring and source of human being - of the being, in fact, of all living things - namely feeling. As he beautifully puts it, "The sick patient, the abandoned lover, the wounded warrior, and the troubadour in love were able to feel." The truth of this is simple and profound; how else may we be said to live, except by feeling?